Fuses and antifuses are common components in conventional integrated circuits. Fuses are commonly formed from a metal or polycide layer which is narrowed down in the region of the fuse. Fuses are then typically blown by applying a voltage or laser to heat the metal or polycide above a melting point, causing the fuse to open and the conductive link. In contrast, an antifuse is a circuit element that is normally open circuited until it is programmed, at which point the antifuse assumes a relatively low resistance. Conventional antifuses are similar in construction to capacitors in that they include a pair of conductive plates separated from each other by a dielectric or insulator. Antifuses are typically characterized by the nature of the dielectric which may be, for example, oxide or nitride. Antifuses are programmed or blown by applying a differential voltage between the plates that is sufficient to break down the dielectric thereby causing the plates to electrically contact each other.
Fuses and antifuses are used in a variety of applications. One such application is to selectively enable certain features of integrated circuits. For example, semiconductor devices are often designed to be operated in multiple modes of operation, with the specific mode of operation programmed after the fabrication of the device has been completed. One method for programming the device is through the use of a fuse or antifuse. More commonly, however, fuses and antifuses are used to perform repairs of integrated circuits, such as in redundancy technology. Repairs of integrated circuits are typically accomplished by blowing the appropriate fuses or antifuses to signal defective portions of the integrated circuit that they should be replaced with redundant circuits. For example, a defective row of memory cells in the array of a dynamic random access memory (DRAM) devices can be replaced with a redundant row of cells provided for that purpose. As demonstrated by this example, redundancy technology can be used to improve the fabrication yield of high-density memory devices, such as DRAM and static random access memory (SRAM) devices, by replacing failed memory cells with spare ones using redundant circuitry activated by programming the fuses or anti fuses.
As previously discussed, antifuses are similar in structure to semiconductor capacitors. Consequently, the fabrication of antifuses can be easily integrated into conventional DRAM device fabrication processes, since, as well known in the art, DRAM devices rely on semiconductor capacitors to store data. However, in devices where capacitors are not typically formed, such as in SRAM devices, integrating the fabrication of antifuses into the conventional process flow is difficult. As a result, fuses are used typically used in SRAM devices rather than antifuses.
Although fuses have been used extensively in semiconductor devices, antifuses provide several advantages over their fuse counterparts. For example, one advantage with antifuses is the ease of programming while the device is on a tester, as opposed to fuses, where the wafers must be transferred to a laser trimmer. Not only does the laser trimming process add time to the entire process, the additional step introduces another point in the process at which catastrophic mistakes can occur. For example, wafers of a lot can be accidentally trimmed using the fuse trimming profile of another lot, or wafers can be rearranged within a lot such that the reordered wafers are trimmed using the incorrect fuse trimming profile. These types of errors typically result in scrapping the mistrimmed wafers.
Additionally, as the size of semiconductor devices decreases, using lasers to blow fuses has become more difficult. That is, as semiconductor devices decrease in size and the degree of integration increases, the critical dimensions, including fuse pitch, become smaller. The availability of lasers suitable to blow the fuse becomes limited since the diameter of the laser beam should not be smaller than the fuse pitch. Thus, the fuse pitch, and the size of semiconductor devices, becomes dictated by minimum diameter of laser beams obtainable by current laser technology.
Moreover, another disadvantage with employing fuses instead of antifuses is related to conventional fuse fabrication processes. As previously discussed, conventional fuse fabrication processes typically form fuses from a polycide layer, which is deposited early in the fabrication process of the device. That is, the polycide layer from which fuses are formed is covered by multiple layers that are formed later in the processing of the device. For semiconductor devices having multiple levels of metallization, such as in SRAM devices, it is becoming very difficult to etch down through the multiple layers of oxide between the levels of metallization to expose the polycide fuses. If the oxide is not sufficiently etched, the fuses may not be completely blown by the laser trimmer, which typically results in malfunction of the device.
Therefore, there is a need for an antifuse structure and method for forming the same that can be integrated into the fabrication processes for devices that typically do not include the formation of semiconductor capacitors.